Topic of The Day: Stability and Control
When the CG is forward of the aerodynamic center, a change in lift causes a nose down pitching moment. This is a stable aircraft. When the CG is aft of the aerodynamic center, a change in lift causes a nose up pitching moment. This is an unstable aircraft.
What affect does a swept wing have on directional stability?
Increases it
When aircraft is rotated about the vertical axis (yaw) the advancing wing presents a larger wing area (less sweep) to the relative wind, thus producing more lift and induced drag slowing it down.
What affect does wing dihedral have on lateral stability?
Increases stability.
When aircraft is rolled about the longitudinal axis, the low wing has a change in relative wind thus increasing AOA and lift, bringing it back up to level.
•Trimmed flight
Equilibrium flight exists when the sum of all forces and the sum of all moments around the center of gravity are equal to zero. An airplane in straight and level flight at a constant velocity is acted upon by four forces: thrust, drag, lift, and weight. When these forces cancel each other out, the airplane is in equilibrium
Trimmed flight exists when the sum of all moments around the center of gravity is equal to zero. In trimmed flight, the sum of the forces may not be equal to zero. For example, an airplane in a constant rate, constant angle of bank turn is in trimmed, but not equilibrium, flight. An airplane in equilibrium flight, however, is always in trimmed flight.
An airplane does not have to be in straight and level flight to be in equilibrium. An airplane that is climbing, but not accelerating or decelerating, (i.e., there are no unbalanced forces) is another example of equilibrium flight
•Relationship between controllability and stability
They are inverse…a designed increase in maneuverability inherently reduces stability and vice versa.
•Function of airplane control surfaces
On aircraft with inboard and outboard ailerons, the outboard ailerons aren’t used at high speed because they cause a twisting moment on the wing
•Proverse roll and adverse yaw
Proverse roll is a rolling/yawing motion into the turn
Adverse yaw is a yawing moment away from the direction of the turn
•Dutch roll
Excessive dihedral effect can lead to “Dutch Roll”
Dihedral effect is your lateral stability (i.e., your tendency to roll back to wings level after upset)
Dutch roll results from relatively weaker positive directional stability as opposed to positive lateral stability. When an aircraft rolls around the longitudinal axis, a sideslip is introduced into the relative wind in the direction of the rolling motion. Strong lateral stability begins to roll the aircraft to level flight. At the same time, somewhat weaker directional stability attempts to correct the sideslip by aligning the aircraft with the perceived relative wind. Since directional stability is weaker than lateral stability for the particular aircraft, the restoring yaw motion lags significantly behind the restoring roll motion. As such, the aircraft passes through level flight as the yawing motion is continuing in the direction of the original roll. At that point, the sideslip is introduced in the opposite direction and the process is reversed.
The most common mechanism of Dutch roll occurrence is a moment of yawing motion which can be caused by any number of factors. As a swept-wing aircraft yaws (to the right, for instance), the left wing becomes less-swept than the right wing in reference to the relative wind. Because of this, the left wing develops more lift than the right wing causing the aircraft to roll to the right. This motion continues until the yaw angle of the aircraft reaches the point where the vertical stabilizer effectively becomes a wind vane and reverses the yawing motion. As the aircraft yaws back to the left, the right wing then becomes less swept than the left resulting in the right wing developing more lift than the left. The aircraft then rolls to the left as the yaw angle again reaches the point where the aircraft wind-vanes back the other direction and the whole process repeats itself. The average duration of a Dutch roll half-cycle is 2 to 3 seconds.
Proper dutch roll damping technique is to apply corrective control wheel deflections as the aircraft rolls through wings-level. The damping technique described in the KC-135R is to “stop the rising wing at the desired bank angle with aileron. As the wing stops, center the control wheel and prepare to stop the other wing from rising”
Manual damping of dutch roll is to be accomplished with lateral (aileron) control. Do not attempt to damp dutch roll manually with the rudder…the sudden reversal of rudder direction at high rudder deflections, due to improper rudder application or abrupt release, can result in overstressing the vertical fin.
Dutch roll is most evident at low indicated airspeeds and high altitudes
Dutch roll oscillations are most likely to arise and amplify at high density altitudes, and thus they are often particularly problematic for high-flying aircraft
The effects of flaps are destabilizing and reduce dihedral effect and reduce dutch roll tendencies
The deflection of flaps causes the inboard sections of the wing to become relatively more effective and these sections have a small spanwise moment arm
Therefore, the changes in wing lift due to sideslip occur closer inboard and the dihedral effect is reduced
